1. Field of the Invention
The field of art to which this invention pertains is a novel pulse width modulated amplifier circuit and in particular to such a circuit which utilizes an integration circuit for eliminating carrier distortion and undesirable oscillations.
2. Description of the Prior Art
Generally, an audio amplifier, particularly a Class-B amplifier comprises a pair of transistors having their respective bases connected to both ends of the secondary winding of a transformer having a center tap input. The emitters of each of said transistors are connected to both ends of the primary winding with a center tap output. The center tap is connected to one end of the power source, and the other end of the power source is connected to the collectors of the transistors. The input signal is applied to both ends of the primary winding of the transformer, while the output signal is extracted from the secondary winding. Such an amplifier is widely used since it is simple in design and low in cost, but this amplifier has low efficiency. This is due to the fact that an unnecessary voltage is applied to one of the two transistors. As is well known, transistors have no capacity to accumulate energy and hence consume the power equal to the product of its voltage and load current.
There have already been proposed various types of pulse width modulated amplifiers (hereinafter referred to as PWM amplifiers) which utilize the power source voltage more effectively than the above low-efficiency amplifiers. Among such known PWM amplifiers is a self oscillation type. This type of amplifier consists of an integration circuit, a hysteresis circuit, an amplifier, and a circuit which picks up the difference between the amplifier output and input signal and feeds the difference signal to said integration circuit. This type of amplifier oscillates without an outside carrier, but it has the defect that the oscillation frequency varies according to the frequency of the input signal and that the sideband of the spectrum interferes with the audio signal to distort the reproduced sound. Another type of PWM amplifier is known in which a square wave is obtained by comparing the carrier signal from the reference oscillation source with the input signal. Such square wave is fed back to the signal input terminal through a low-pass filter. This type of amplifier, however, is high in carrier signal frequency, and the low-pass filter is unable to remove the carrier sufficiently, so that there is the danger that the loop will oscillate in the frequency range of the carrier signal. If the cut-off frequency of the low-pass filter is lowered enough to eliminate carrier signal, it may follow that no satisfactory loop gain is obtained in the audio signal. Also, if the carrier is not sufficiently removed, the side bands may mix with the demodulated signal to distort the reproduced audio.